Today high voltage (HV) cables are typically made of cross-linked polyethylene (XLPE). Cross-linking can be achieved by the addition of low amounts of dicumyl peroxide (DCP) in a low density polyethylene (LDPE) base material.
Factory, sea or flexible HV power cable joints are normally manufactured using cross-linkable tapes wound around the conductor in several layers. These layers may consist of an innermost layer that may be made of a semiconducting material, an insulating layer arranged around the semiconducting layer, and an outermost layer also made of a semiconducting material. The insulating layer may for example be made of XLPE and the semiconducting layers may be made of carbon-black filled XLPE. The tapes are manually lapped between the two cable ends of the tapered insulation of the two cables that are to be jointed. Each of these three layers is then cured under heat and pressure in a separate step so that the tapes melt together forming a homogeneous material without any voids.
The tapes used for the insulation system of the factory joints and the factory joints as such are manufactured in the cable factory under strict cleanness conditions. The same applies also in case the insulation system of a defected power cable is to be restored. The manual jointing or restoration procedure is lengthy and vulnerable to contamination. It may require up to four days, or more, of manufacturing time, including the required crosslinking process, in three shift work schedule. Moreover, a person performing the jointing/restoration procedure must be trained for several years before being authorised to perform the jointing. Furthermore, additional time is essential for heat treatment in order to degas polar by-products of the crosslinking reaction, known to impair the insulation's performance. The latter process can last from few days up to weeks.